Turkish helicopters have been flying into Iraq as part of the offensive

It said the incident happened "due to an unknown reason". Earlier, Kurdistan Workers' Party (PKK) rebels claimed to have shot down a Turkish helicopter.

Turkey says more than 112 PKK militants have been killed as well as 15 of its own soldiers in three days of fighting.

Iraq has urged Turkey to withdraw its forces and hold talks on the PKK issue.

And the US has called on Turkey to keep its military campaign as short as possible.

The Turkish authorities launched the cross-border attack on Thursday night, after accusing the Iraqi government of failing to stop members of the PKK from using the area as a safe haven.

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More than 30,000 people have been killed since the PKK began fighting for a Kurdish homeland in south-eastern Turkey in 1984.

The US, the EU and Turkey consider the PKK to be a terrorist organisation.

The PKK disputes that, claiming that it has killed 47 Turkish troops.

Ahmed Danees, head of foreign relations for the PKK, said his group had on Saturday shot down a Turkish Cobra attack helicopter.

He added that this had happened in the remote Chamsku area, close to the border.

Escalation threat

In its latest internet statement, the Turkish military says its ground troops, supported by fighter jets and helicopter gunships, are fighting PKK separatists in three locations inside northern Iraq.

It says that that over 60 PKK targets, including shelters and weapons stores, have been destroyed and that PKK militant are retreating to the south.

The BBC's Sarah Rainsford in Turkey says that if that is true and Turkish troops follow them, the danger of the conflict spreading will increase.

Iraqi-Kurdish Peshmerga have already vowed massive resistance if local civilians are harmed.

Political measures

Iraq's government on Sunday renewed a call for Turkey to remove its troops from Iraqi territory "as soon as possible," calling the offensive "a threat to the stability of the region and a violation of Iraq's sovereignty".

Gates urged Turkey to respect Iraq's sovereignty

Speaking at the end of a visit to Australia, US Defence Secretary Robert Gates said military strikes by Turkey would not be enough on their own to end the long-running dispute with Kurdish rebels.

He said Ankara should employ political and economic measures to isolate the PKK and erode its support base.

He said America's experiences in Iraq and Afghanistan showed that military muscle should be complemented by efforts to address grievances held by minority groups.

"These economic and political measures are really important because after a certain point people become inured to military attacks," he said.

"And if you don't blend them with these kinds of non-military initiatives then at a certain point the military efforts become less and less effective."

The American defence secretary will deliver his advice in person when he travels to Ankara next week.